Rivers Piddle and Frome, and Poole Harbour
Rivers Piddle and Frome, and Poole Harbour
Rivers Piddle and Frome, and Poole Harbour
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Alan Walker, Mike Pawson<br />
Case Study Description<br />
<strong>Rivers</strong> <strong>Piddle</strong> <strong>and</strong> <strong>Frome</strong>,<br />
<strong>and</strong> <strong>Poole</strong> <strong>Harbour</strong><br />
Centre for Environment, Fisheries <strong>and</strong> Aquaculture Science (Cefas)<br />
Lowestoft Laboratory<br />
Pakefield Road, Lowestoft, Suffolk NR33 0HT.<br />
Tel + 44 1502 524351, Fax + 44 (0) 1502 526351,<br />
Mike.Pawson@cefas.co.uk; Alan.Walker@cefas.co.uk<br />
1 Introduction<br />
The River <strong>Frome</strong> is a chalk stream in SW Engl<strong>and</strong>, with a main stem of 48 km in length. It<br />
rises in the Dorset Downs at Evershot, <strong>and</strong> passes through Maiden Newton, Dorchester, West<br />
Stafford <strong>and</strong> Woodsford. A key tributary of the River <strong>Frome</strong> is Tadnoll Brook, which enters<br />
the river 10 km upstream of the tidal influence (Figure 1). The River <strong>Piddle</strong> rises in Alton<br />
Pancras <strong>and</strong> flows for ~30 km south <strong>and</strong> then south-east, approximately parallel to the River<br />
<strong>Frome</strong>. Two key tributaries of the River <strong>Piddle</strong> are the Bere Stream <strong>and</strong> Devils Brook. The<br />
combined <strong>Frome</strong> <strong>and</strong> <strong>Piddle</strong> catchment area is 454 km².<br />
Both rivers drain into <strong>Poole</strong> <strong>Harbour</strong>, which has an area of 38 km². The harbour is extremely<br />
shallow (average depth: 48 cm), with one main dredged channel, <strong>and</strong> is characterized by<br />
extensive mud flat <strong>and</strong> salt marsh habitats, as well muddy shores, s<strong>and</strong>y shores <strong>and</strong> seagrass<br />
meadows.<br />
There are no significant barriers to eel migration on either the <strong>Frome</strong> or the <strong>Piddle</strong>, other than<br />
possibly the flood alleviation scheme in the upper <strong>Piddle</strong> (see below). The surrounding l<strong>and</strong><br />
usage is predominately agricultural with no industry or major settlements within their<br />
catchments. As a consequence, water quality remains very good <strong>and</strong> both rivers have the fauna<br />
<strong>and</strong> flora typical of lowl<strong>and</strong> chalk streams.<br />
A flood alleviation scheme was completed in October 2005 at <strong>Piddle</strong>trenthide (2.5 km<br />
downstream from the source of the River <strong>Piddle</strong>). Floodwater bypasses the village via a<br />
culvert 560 m in length. Eels were present in this area in 2003, both above (density 0.02<br />
eels.m -2 , n = 7) <strong>and</strong> below the village (density 0.14 eels.m -2 , n = 58). There is no information<br />
regarding what, if any, effect the culvert may have on the eel population in this part of the<br />
upper catchment. The rivers <strong>Piddle</strong> <strong>and</strong> <strong>Frome</strong> have been extensively studied for their trout<br />
<strong>and</strong> salmon populations, fish habitats <strong>and</strong> invertebrates. Angling on both rivers is very<br />
popular, especially for salmon, trout <strong>and</strong> grayling.<br />
There are two silver eel racks, one on the River <strong>Piddle</strong> <strong>and</strong> one on the <strong>Frome</strong>. The <strong>Frome</strong> rack<br />
tends only to be run for research purposes. The River <strong>Piddle</strong> rack operates each year during<br />
the silver eel run <strong>and</strong> the catch is sold for UK consumption. This trap was fitted with new rack<br />
in 2000 with 10 mm bar spacings to ensure all male silver eels were caught.<br />
This document is part of the Report of FP6-project FP6-022488 ‘Restoration of the European eel<br />
population; pilot studies for a scientific framework in support of sustainable management.’<br />
lime - Study Leading to Informed Management of Eels<br />
Sixth Framework Programme, Priority 8.1, Policy-oriented research, Scientific Support to Policies.
<strong>Poole</strong> <strong>Harbour</strong> supports a fyke net fishery. Local eel fishermen claim that stocks <strong>and</strong> fisheries<br />
have declined <strong>and</strong> the average size of eels in catches has fallen over the last 20-30 years. The<br />
situation was investigated for the Environment Agency (Knights & White, 1997) <strong>and</strong> a<br />
summary of key points <strong>and</strong> conclusions is given below.<br />
Fyke nets were introduced into the <strong>Harbour</strong> fishery in the 1950s <strong>and</strong> catches in the 1960-70s<br />
were about 30 t year -1 (Morrice, 1989). At the end of the 1960s, five ‘punts' were laying up to<br />
650 fyke-net ends per night. Increasing effort (as much as a tenfold increase in numbers of<br />
ends deployed per day) was needed to maintain catches in the early 1980s. Subsequent<br />
declines in catches have, it is claimed, led to reduced numbers of fishermen <strong>and</strong> hence total<br />
effort since the mid-1980s. Given the area of <strong>Poole</strong> <strong>Harbour</strong> as 38 km 2 , <strong>and</strong> extrapolating from<br />
estimated total commercial yields, Knights & White (1997) calculated that catches could have<br />
fallen from about 7.9 to 1.3 kg ha -1 year -1 in the between the 1960-70s <strong>and</strong> the 1990s, a<br />
decline of 80-85%. Although firm evidence is lacking, growth-overfishing is implicated,<br />
probably exacerbated by reduced recruitment.<br />
In relation to more recent changes, quantitative information on fishery yields was first<br />
collected by the former NRA in 1988, licensing of the fishery began in 1991, <strong>and</strong> catch returns<br />
are probably inaccurate. In 1988, the declared catch was 17,045 kg (including 1270 kg caught<br />
by a tidal seine net), but there are no records for fishing effort. Between 1991-96, the average<br />
catch was 19.6 (range 10.9-28.0) kg fyke-net-end -1 year -1 , shared between 2 to 8 licensees,<br />
with no apparent trend.<br />
Figure 1 The River <strong>Piddle</strong>, River <strong>Frome</strong> <strong>and</strong> <strong>Poole</strong> <strong>Harbour</strong>.<br />
2 Fishing capacity<br />
There is no limit on fishing capacity<br />
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3 Fishing effort<br />
There is no limit on fishing effort, the total fishing effort recorded in 2005 was 800 fyke net<br />
days.<br />
4 Catches <strong>and</strong> L<strong>and</strong>ings<br />
In 2005 the declared catch was 300 kg of yellow eel <strong>and</strong> 126 kg of silver eel.<br />
5 Catch per Unit of Effort<br />
In 2005 the CPUE was 0.53 kg per fyke net per day<br />
6 Scientific surveys of the stock<br />
6.1 Recruitment surveys<br />
No surveys on glass eel/elver recruitment have been carried out for the <strong>Piddle</strong>/<strong>Frome</strong> system.<br />
6.2 Yellow eel surveys<br />
Electric fishing surveys were conducted throughout the <strong>Piddle</strong> in 1977, 1999 (16 sites), 2003<br />
(20) <strong>and</strong> 2004 (5). The data set for 1977, based on two single-pass runs of 14 km one month<br />
apart, does not include eels
6.3 Silver eel surveys<br />
Silver eel were measured for length <strong>and</strong> weight from catches at licensed fixed eel racks fished<br />
on a varying number of nights over the period of the silver eel run from September to<br />
December, in 1977 (<strong>Piddle</strong>) <strong>and</strong> 2003–2004 (<strong>Piddle</strong> <strong>and</strong> <strong>Frome</strong>).<br />
7 Catch composition by age <strong>and</strong> length<br />
‘Catch’ composition by age <strong>and</strong> length are available for all the electric fishing <strong>and</strong> fyke netting<br />
(yellows), <strong>and</strong> eel rack (silvers) surveys.<br />
8 Other biological sampling (age <strong>and</strong> growth, weight, sex,<br />
maturity, fecundity).<br />
See Section 6, above.<br />
9 Literature references.<br />
Knights B. & White E.M. (1997) An appraisal of stocking strategies for the European eel,<br />
Anguilla anguilla L. In Stocking <strong>and</strong> Introduction of Fish (ed. I.G. Cowx), pp. 121-140.<br />
Fishing News Books, Oxford.<br />
Knights, B., Bark, A., Ball, M., Williams, F., Winter, E. <strong>and</strong> S. Dunn (2001). Eel <strong>and</strong> elver<br />
stocks in Engl<strong>and</strong> <strong>and</strong> Wales – status <strong>and</strong> management options. Environmental Agency,<br />
Research <strong>and</strong> Development Technical Report W248. 294 pp.<br />
Morrice, C.P. (1989) Eel fisheries in the United Kingdom. MAFF Internal Report No. 18,<br />
33pp. MAFF, Lowestoft, Engl<strong>and</strong><br />
10 Application of the Models<br />
For the purposes of testing models in SLIME, it was decided to focus on the freshwater<br />
portion of the <strong>Piddle</strong> catchment <strong>and</strong> exclude the estuarine <strong>Piddle</strong> <strong>Harbour</strong>, since the latter is<br />
the site of a mixed stock eel fishery (with the River <strong>Frome</strong>).<br />
10.1 SMEP<br />
Two models were applied to the <strong>Piddle</strong> dataset; SMEP <strong>and</strong> GlobAng.<br />
The <strong>Piddle</strong> population was appropriate for modelling with SMEP because catchment data<br />
were readily available, along with eel population data sufficient to inform the parameters file.<br />
10.1.1 Parameter Setting<br />
At the time of the workshop, SMEP applied growth according to the Von Bertalanffy model,<br />
<strong>and</strong> ‘UK eel’ default values for L∞ <strong>and</strong> k were used. It was recognised, however, that the VB<br />
model is probably not appropriate for simulating growth of eel, <strong>and</strong> this is an area of continued<br />
model development.<br />
Recruitment: No direct recruitment measures were available for the <strong>Piddle</strong>, but a trend was<br />
applied to simulate changes in CPUE of UK glass eels fisheries since 1980. Elvers recruit to<br />
the <strong>Piddle</strong> after a period of growth in <strong>Poole</strong> <strong>Harbour</strong>: the mean length of recruits was<br />
increased above the default value for glass eel in order to reflect this.<br />
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Carrying Capacity (biomass): maximum densities observed for streams were converted to<br />
biomass, although these may still be well below maximum possible biomass of yellow eel<br />
st<strong>and</strong>ing stock. Both the concept, <strong>and</strong> representative values, of carrying capacity for eel<br />
require further exploration.<br />
10.1.2 Population Data<br />
Preliminary estimates based on mark-recapture exercises suggest that recent silver eel<br />
escapement from the <strong>Piddle</strong> is in the region of 6000 eels per year.<br />
The most complete data set for yellow eel was for 2003, so this was used as a reference year,<br />
against which to compare the model outputs in terms of the yellow eel production.<br />
10.1.3 Model Outputs<br />
Modelling with SMEP suggested that a recruitment of 1.2 million elvers would be required to<br />
produce an average annual silver eel escapement of 6000 eels. Based on this level of presentday<br />
recruitment, the SMEP model suggested that historic (pre-1980) recruitment <strong>and</strong> silver eel<br />
escapement would have been 4.8 million elvers <strong>and</strong> 24,000 silver eels, respectively. However,<br />
both modelled silver eel runs were dominated by male eels (97-100%), whereas catches of<br />
silver eels since 1999 have had much higher proportions of females (46-99%). Furthermore,<br />
SMEP required yellow eel densities in the three reaches that were orders of magnitude higher<br />
than observed in order to produce this level of silver eel escapement.<br />
Calibrating the model with the yellow eel densities observed in 2003 suggested a recruitment<br />
of about 70,000 but a silver eel run of only 400+.<br />
10.1.4 Conclusion<br />
SMEP has definite potential for modelling eel populations <strong>and</strong> silver eel output, but will<br />
require considerable further parameter testing <strong>and</strong> development to cope with application to<br />
data-poor rivers. Describing the catchment in reaches, setting an appropriate level of<br />
recruitment where no actual data exist, <strong>and</strong> using parameter settings suitable for the river type<br />
appear be the key constraints to useful model outcome. These three aspects are all featured in<br />
the ongoing development of SMEP.<br />
10.2 GlobAng<br />
GlobAng is designed to perform simulations of eel population dynamics within a<br />
hydrographical network. After calibration with real field data, the model is able to evaluate<br />
silver eel escapement in response to a variety of management scenarios, especially when<br />
spatial dimension is important. The model integrates ageing, recruitment, sexual<br />
differentiation, silvering, natural mortality <strong>and</strong> for the first time migration within a watershed.<br />
Fishing mortality <strong>and</strong> impacts of migration barriers can also be simulated.<br />
Similar data were applied to the GlobAng model as were applied to SMEP <strong>and</strong> the same<br />
recruitment <strong>and</strong> carrying capacity comments made under SMEP apply here. However, as<br />
GlobAng requires reaches of equal length, the three SMEP reaches were split into seven, each<br />
of nearly 10 km long. Also, as GlobAng is an age-related model, the observed length<br />
distributions were transformed to age distributions using a length-age key developed from the<br />
2003 data.<br />
10.2.1 Model Outputs<br />
Calibration of the model with the recruitment trend <strong>and</strong> the observed yellow eel population in<br />
the three reaches in 2003 suggested a historic (pre-1980) recruitment level of around 550,000<br />
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<strong>and</strong> an annual escapement of 6250 silver eels, most of which were female. According to the<br />
CPUE trend, therefore, the present-day recruitment would be about 140,000 elvers, which the<br />
model suggests should produce about 3000 silver eels per year, most of which will be females.<br />
The model overestimated the abundance of young eels, especially in the lower reaches.<br />
However, the model set-up required recruitment from <strong>Poole</strong> <strong>Harbour</strong> to reach 1 at elver age 2,<br />
whereas, as suggested by PL <strong>and</strong> others before, <strong>Poole</strong> <strong>Harbour</strong> may be acting as a recruitment<br />
sink from which elvers migrate upstream at a range of ages.<br />
The model underestimated both the abundance of older eels throughout, <strong>and</strong> the average age<br />
of silver eels, at 4 <strong>and</strong> 10 years for males <strong>and</strong> females, whereas the average age of silver eels<br />
sampled in 2000 to 2004 was 17 years, <strong>and</strong> ranged from 9 to 29 years old. This suggests that<br />
the growth <strong>and</strong> probably maturation modelling were inappropriate for the <strong>Piddle</strong> eel<br />
population.<br />
10.2.2 Conclusion<br />
GlobAng predicted a level of present-day silver eel escapement that, while it was half that of<br />
the mark-recapture estimate, was a much better prediction than that provided by SMEP.<br />
Improvements are needed to simulate a more effective colonisation of the catchment, age<br />
structure of silver eels <strong>and</strong> to shift the sex ratio in favour of males. It was noted, however, that<br />
the observed densities did not correspond to the typical case of diffusive colonisation with<br />
higher abundance in the reaches closest to the estuary. Perhaps, therefore, a more<br />
comprehensive survey programme would facilitate better representation of the yellow eel<br />
stocks in each reach, <strong>and</strong> allow GlobAng to better model the <strong>Piddle</strong> eel stock.<br />
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